The present invention relates generally to filters, and more specifically to a method and apparatus for active damping of LC filters.
Power electronic systems such as converters are typically used to supply power to various loads. It is desirable that the voltage or current supplied to such loads is purely sinusoidal. To obtain sinusoidal waveforms, most power electronic systems use filters. For example, in a uninterrupted power supply (UPS) system, a second order filter is typically used in conjunction with a three phase inverter to supply sinusoidal voltage or current to the load.
Filters are usually implemented using inductors and capacitors. One of the important characteristics of an LC filter is resonance. Resonance is defined as the condition that exists when the inductive reactance and the capacitive reactance of the LC filter are of equal magnitude and the corresponding frequency is known as resonant frequency. Mathematically, resonant frequency equals:   fr  =      π          2      ⁢              LC            
wherein L is the value of the inductor, C is the value of the capacitor and fr is the resonant frequency. At resonant frequency, the impedance of the filter is minimal which in turn causes an undesirably large voltage to be delivered to the load. To prevent such large voltages from being delivered to a load, the filters are usually damped.
One common method by which a filter can be damped is by coupling a resistance in series or parallel with the inductor or capacitor. A problem with this solution is that undesirably large amounts of power are lost during normal filter operations. Additionally, the filter circuit becomes more bulky because the resistor typically consumes significant space.
An alternative method to damp filters is to reduce the gain in a controller which is usually present within the power electronics system. In this method, the overall speed of the system can be reduced.
Therefore, what is desired is a method and apparatus for damping filters while minimizing power and speed losses.
Briefly, in accordance with one embodiment of the invention, a method for damping an LC filter coupled to a converter comprises sensing a filter signal of the LC filter and processing the filter signal to generate a corresponding feedback signal. The feedback signal is subtracted from a controller signal to generate a difference signal. The difference signal is used for damping the LC filter.
Another aspect of the invention provides a damping device for damping an LC filter coupled to a converter. The damping device comprises a feedback block configured for processing a filter signal from the LC filter for generating a feedback signal. The damping device also comprises a subtractor configured for using the feedback signal and a controller signal for generating a difference signal. The difference signal is used for damping the LC filter.
Another aspect of the invention provides a system for damping an LC filter. The system is coupled to a converter and comprises means for obtaining a filter signal from the LC filter and means for processing the filter signal to generate a corresponding feedback signal. The system further comprises means for subtracting the feedback signal from a controller signal to generate a difference signal and means for using the difference signal for damping the LC filter.
Another aspect of the invention provides a computer-readable medium storing computer instructions for instructing a computer system to formulate a control signal for damping an LC filter. The computer instructions include obtaining a filter signal of the LC filter and processing the filter signal to generate a corresponding feedback signal. The computer instructions further include subtracting the feedback signal from a controller signal to generate a difference signal and using the difference signal to generate the control signal for damping the LC filter.